This invention relates to semiconductor integrated circuit fabrication, and in particular to a means of insuring proper focusing during optical projection printing or other processing of the semiconductor wafer.
With the desire for increasing circuit density in semiconductor IC fabrication, the need for fine-line circuit pattern generation has become acute. The typical method of generating patterns in the wafer involves forming a photoresist layer over a major surface of the wafer and exposing the layer to light through a mask. The photoresist is then developed to expose the wafer surface in the desired pattern. One generally accepted method of exposing the photoresist layer is optical projection printing where the mask is imaged onto the photoresist layer rather than being in direct contact therewith. Step and repeat printers and scanning printers, for example, provide this type of pattern generation. (For an example of the latter, see U.S. Pat. No. 3,819,265 issued to Feldman et al.)
In order to insure fine-line definition, it is usually necessary during projection printing to provide some means for monitoring the position of the wafer to insure that the mask is precisely focused onto the photoresist layer throughout the entire printing operation. A typical method of providing this focus tracking involves focusing laser light at a grazing angle to the photoresist layer and then imaging the reflected light onto a split photodiode. The position of the imaged light on the photodiode depends on the vertical position of the wafer, and movement of the light position generates signals which are coupled to means for adjusting the position of the wafer. (See, e.g., U.S. Pat. No. 3,819,265 cited above.)
While such focus tracking systems are generally adequate, accuracy can be affected by variations in reflectivity along the wafer surface due to differences in topography or other causes.
In addition to projection printing, other steps in semiconductor fabrication may require focus tracking. For example, it may be desirable to examine automatically a pattern on a wafer or mask to measure linewidth. Further, as an alternative to optical projection printing, an electron beam exposure system might be employed for pattern generation. Thus, focus tracking is desirable wherever a wafer or mask surface is required to be in a certain plane.
It is, therefore, an object of the invention to provide focus tracking of a semiconductor wafer or mask which provides high sensitivity so that an accurate measure of wafer or mask height is obtained, but which is relatively insensitive to variations in wafer or mask reflectivity.